The present invention relates to telephone communication systems and, more particularly, to a circuit for determining when a telephone unit of a telephone subscriber is in an off-hook condition.
Automated telephone communication systems for reporting information to a central location via existing telephone lines are well known. One such system is disclosed in U.S. Pat. No. 4,412,292, issued Oct. 25, 1983, to Sedam et al. The Sedam et al system monitors a soft drink vending machine and transmits data relating to the operation of the vending machine to a central computer at scheduled times. The system is connected to a telephone line which provides telephone service to the premises upon which the vending machine is located. The telephone communication system notifies the central computer upon occurrence of any of a number of alarm conditions, such as illegal entry, machine jamming, improper changer operation, sale of all items, loss of electrical power, and the like. Additionally, the system may call the central computer on a scheduled basis to provide inventory information.
In another type of automated telephone communication system, shown in U.S. Pat. No. 4,455,453, issued June 19, 1984, to Parasekvakos et al, a power meter is automatically read by circuitry located at the meter site. The communication system, also at the meter site, initiates a telephone call to a central computer at predetermined call-back times and transmits power consumption data.
Commonly, such an automated telephone communication system is not connected to a dedicated telephone line. Rather, a telephone line normally used for other purposes is accessed briefly by the system to report the data to the central computer. In the case of a system reporting a power meter reading, the communication system is connected to the power consumer's telephone line. A report from such an automated system may, for example, take less than a minute to transmit via a modem connected to the telephone line, and such a transmission can be scheduled for the early morning hours when other use of the telephone line is unlikely.
It will be appreciated that by connecting the automated telephone communication system to the telephone line of the subscriber rather than to a dedicated telephone line, significant cost savings are achieved. It will also be appreciated that should the telephone subscriber experience interference with his free use of his telephone, an automated telephone communication system is unlikely to gain his acceptance.
In the Parasekvacos et al system, the communication circuitry checks prior to a transmission to the central computer to see if any of the telephone units on the line are in an off-hook condition. Typically, the voltage across a pair of telephone conductors is approximately 48 volts DC when there are no telephone units across the conductors in an off-hook state. When a telephone handset of one of the units is taken off-hook, however, the unit then draws 80-180 m.a. and the voltage across the telephone conductors at the unit drops to less than 15 volts DC. The Parasekvacos et al circuit checks the voltage across the telephone conductors by means of an electronic circuit. A Zener diode in the circuit is connected across the telephone conductors to supply a gate signal to an SCR, turning the SCR on and initiating operation of the data transmission circuitry. If a telephone unit, connected across the conductors is in an off-hook condition and has thereby reduced the voltage across the conductors, however, an insufficient voltage will be supplied to the gate of the SCR. As a result, the transmission circuitry will not operate, and the telephone call then being made will not be adversely affected.
Several problems are not addressed by prior art devices. For example, it may happen that the polarity of the DC voltage on the telephone conductors is reversed inadvertently when the conductors are connected to the automated telephone communication system. With some prior art systems, this may result in a failure to properly detect off-hook conditions. Additionally, while a transmission to the central computer is taking place, a subscriber may pick up a telephone unit handset and attempt to place a call. This may adversely affect the transmission of data to the central computer and, additionally, it undoubtedly will annoy the subscriber since he will not be able to place his call. It will be appreciated that, in the case of emergencies, it is especially important that a telephone unit remain operative. Further, it may be desirable to be able to detect a ring signal on the telephone conductors.
It is seen, therefore, that there is a need for a dependable monitor circuit for checking the availability of a telephone line prior to the transmission of data over the line, for a monitor circuit which monitors the continued availability of the line during the transmission of data, and for such a monitor circuit which detects a ring signal.